1. Field of the Invention
The present invention relates to a printing apparatus in which a printhead incorporates a temperature sensor.
2. Description of the Related Art
The printhead of inkjet printing apparatuses that are formed from a semiconductor integrated circuit are known to suffer an increase in ink discharge amount along with the temperature rise of the printhead. High reproducibility and color stability of printed images even during continuous printing is required from inkjet printing apparatuses. This has prompted development of a technique for precisely controlling the driving voltage and driving pulse of a printhead (Japanese Patent Laid-Open No. 2007-69575). According to this technique, the signal processing circuit of the printing apparatus adjusts the driving conditions (driving voltage and driving pulse) of the printhead based on temperature data detected by a temperature sensor incorporated in the printhead, and performs control in order to make the ink discharge amount uniform.
However, during a printing operation, high-frequency noise from a digital signal such as a print data signal is combined with an output signal from the temperature sensor incorporated in the printhead, and inhibits accurate temperature detection. Hence, the period during which the driving conditions (driving voltage and driving pulse) of the printhead can be controlled is limited to the interval between printing operations (periods when no ink is discharged at the sheet end or the like).
In general, a temperature detection arrangement such as the temperature sensor incorporated in the printhead often uses a diode temperature sensor arrangement which detects the forward voltage of a forward biased p-n junction. It is therefore necessary to detect a small voltage change complying with the temperature characteristic (−2 mV/° C.) of the forward voltage of the p-n junction. In the semiconductor integrated circuit which supports the temperature sensor, digital signals such as a data signal and clock signal are supplied next to the temperature detection signal line. Noise from these digital signals is combined with the temperature detection signal, resulting in error in detected temperatures.
Japanese Patent Laid-Open No. 8-136356 describes an arrangement which can reduce an offset generated in a detected voltage by restricting, to a predetermined current range, a DC bias current Ibias for forward biasing the p-n junction of a diode temperature sensor, and setting the operation resistance of the diode to a predetermined value. To set the operation resistance to a predetermined value, a resistor is series-connected to the diode. However, in a diode temperature sensor formed as a substrate transistor structure, the DC bias current flows through the substrate, and may raise the substrate potential to cause latch-up. To prevent this, the DC bias current needs to be minimized. Further, series-connecting the resistor to the diode is not desirable because the detection sensitivity for the forward voltage of the diode upon a temperature change decreases and thus the S/N ratio drops.
Japanese Patent Laid-Open No. 2005-147895 describes an arrangement in which resistors are interposed between the anode of a diode temperature sensor and the power supply and between the cathode and GND. This arrangement can reduce combined noise by equalizing resistance values. However, a diode temperature sensor formed from a forward biased p-n junction in a semiconductor integrated circuit has a transistor structure. Especially in a semiconductor integrated circuit using a normal CMOS process, a substrate transistor can form a forward biased p-n junction. For a p-type substrate, a special process needs to be introduced to form a diode temperature sensor floated from GND. Also, Japanese Patent Laid-Open No. 8-136356 does not particularly mention a concrete arrangement position of the resistor.
Japanese Patent Laid-Open No. 2002-280556 describes an arrangement in which capacitors are interposed between the cathode of a diode temperature sensor and the substrate of a semiconductor element and between the anode and the substrate, and the two capacitors have the same the capacitance value. However, the capacitance value of a capacitor formable in a semiconductor integrated circuit is as small as about several pF, and is not enough to reduce combined noise.
Japanese Patent No. 3509623 describes an arrangement in which an RC filter is formed in a semiconductor chip with respect to the read signal line of a semiconductor temperature sensor to remove noise. The resistor of the RC filter is series-connected to a temperature sensor element, and a capacitor is parallel-connected. The capacitor is formed on a gate oxide film on a contact pad. However, noise combined with a diode temperature sensor has a vertically asymmetrical voltage waveform due to nonlinearity of the diode. Despite smoothing by the RC filter, a DC component is generated as an offset voltage, resulting in a temperature detection error. In addition, series-connecting the resistor to the diode temperature sensor is not desirable because the temperature detection sensitivity drops.